1. Field of the Invention
The present invention relates to a catalytic cracking process and apparatus therefor.
2. Description of Related Art
In conventional catalytic cracking, particles of cracking catalyst are contacted in a reaction zone with a hydrocarbon feed at catalyst temperatures which are sufficiently high to cause a reasonable percentage conversion of the feed with an acceptable amount of by-product such as coke and normally gaseous products. The coke is deposited on the catalyst particles and is removed therefrom by contacting the coked catalyst particles with an oxygen-containing gas in a regenerator whereby coke is removed by exothermic oxidation which heats the catalyst particles as they are thus regenerated. The heated regenerated particles are contacted with further amounts of hydrocarbon feed.
A factor determining the temperature of the heated regenerated particles is the amount of coke-removal in the regenerator. If the feed is of the type which causes relatively large coke deposits, the oxidation of the coke can result in heated regenerated catalyst particles whose temperature is so high that upon contact with the feed, coke (and light hydrocarbon gases) are formed as by-products in amounts which are significantly detrimental to the economics of the catalytic cracking operation. It is accepted that coke must be removed from used catalyst particles in order to regenerate the catalytic properties of the particles. It will be appreciated from the foregoing that in catalyst cracking operations wherein the heat produced by coke oxidation results in excessively high regenerated catalyst temperatures, there can be detriment to the economics of the catalytic cracking operation.
It is known to cool catalyst particles circulating in a fluidized catalytic cracking unit ("FCCU"). For example, U.S. Pat. Nos. 4,820,404 and 4,917,790 disclose cooling spent or used catalyst from the catalyst stripper by indirect heat exchange with a cooling fluid to moderate the temperature in part of the regenerator.
U.S. Pat. No. 4,971,681 discloses the use of flue gas from the cyclone section of the regenerator to indirectly heat the spent catalyst in the stripping zone of the reactor. The regenerator flue gas contains a dilute concentration of catalyst particles in it. The flue gas contains carbon monoxide which when mixed with oxygen burns to carbon dioxide inside the heat exchanger positioned in the catalyst stripper section of the reactor.
U.S. Pat. No. 5,062,945 discloses the use of hot regenerator flue gas to heat the catalyst stripper section. This patent does not recognize the use of regenerated catalyst from the dense catalyst phase of the regenerator to heat the spent catalyst for stripping.